1. Field of the Invention
The present invention relates to an optical circuit device and, more particularly, to an optical circuit device having a multi/demultiplexing function suitable for wavelength division multiplexing optical communication.
2. Description of the Prior Art
With recent increases in information transmission capacity, wavelength division multiplexing (WDM) optical communication that transmits different pieces of information using different wavelengths has come to the forefront of the technology. Research institutes are engaged in vigorous research and development of devices for multiplexing and demultiplexing wavelengths as key devices in WDM.
Of such devices, arrayed-waveguide gratings (AWGS) have been developed by various research institutes, and have recently exhibited great technical advances. An AWG has satisfactory characteristics for wavelength multi/demultiplexing on a single device. However, the device size is large, and the cost is high. For this reason, the AWG can be suitably used as a device for a trunk line system, but is not suitable for a subscriber system.
A Mach-Zehnder (MZ) optical multi/demultiplexer and an optical demultiplexer using directional coupler have also been proposed. In the former device, diffraction gratings are formed on two arms of a balanced MZ interferometer to implement a multi/demultiplexing function. This device is described in detail in Japanese Unexamined Patent Publication Nos. 9-61649 and 1-172924. The latter device is described in detail in I. Baumann et al., "Compact All-Fiber Add-Drop-Multiplexer Using Fiber Bragg Gratings" (IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 8, No. 10, pp. 1331-1333 (1996).
FIG. 1 is a plan view showing the schematic arrangement of a conventional optical demultiplexer using a directional coupler. In the optical device shown in FIG. 1, first and second fibers 68 and 69 as single-mode fibers are held nearby on a substrate 70 to form a directional coupling section 71.
A diffraction grating 72 is formed on the directional coupling section 71 by using optically induced refractive index modulation upon ultraviolet irradiation, thereby implementing an optical demultiplexer.
When light beams having different wavelengths are inputted through an input port 73, since a length L of the directional coupling section 71 is equal to the complete coupling length, light of the wavelength transmitted through the diffraction grating 72 is outputted from an output port 76.
In this case, the "complete coupling length" is the minimum length that is required to completely couple light inputted to the first optical waveguide (first fiber 68) with the second optical waveguide (second fiber 69).
Letting .kappa.c be the coupling coefficient of the directional coupling section 71, a complete coupling length L1 is given by: EQU L1=.pi./2/.kappa.c (1)
Light having the wavelength reflected by the diffraction grating 72 is output from a demultiplexing port 74.
FIGS. 2 and 3 are graphs showing light intensity/wavelength characteristics to explain the operation of the conventional optical demultiplexer. FIGS. 2 and 3 respectively show a demultiplexing port output waveform 79 and an output port output waveform 80 at the demultiplexing port 74 and an output port 76 when broadband light is input through the input port 73.
A Bragg wavelength .lambda.B of the diffraction grating 72 is 1,536 nm. In the conventional optical circuit device, the diffraction grating 72 is placed at a distance La from a start position 77 of the directional coupling section 71 to minimize reflection loss of light having the wavelength .lambda.B at the input port 73.
More specifically, the position La of the diffraction grating 72 is set so that: EQU 4.times..kappa.c.times.La+d.phi.=.pi. (2)
where d.phi. is the phase difference between the even and odd modes of light having the wavelength .lambda.B reflected by the diffraction grating 72. Equation (2) indicates that the phase difference between the even and odd modes of light having the wavelength .lambda.B becomes .pi. when the light is reflected by the diffraction grating and returns to the start position 77.
In this case, the even mode indicates a case in which the phase difference between light beams propagating in two waveguides is 0, whereas the odd mode indicates that the phase difference between light beams propagating in two waveguides is .pi..
Ideally, reflection loss at the input port 73 can be perfectly suppressed by satisfying equation (2). In the prior art, L1=10 mm, LG=2.5 mm, .lambda.B=1536 nm, and the refractive index modulation of the diffraction grating 72: .DELTA.n=1.3.times.10.sup.-3, and the diffraction grating is formed at La=4.7 mm.
In the conventional optical circuit device, to add a multiplexing function, light having the wavelength .lambda.B inputted through a port 75 must be multiplexed with the output from the output port 76.
The reflection loss at the port 75 is, however, large because the phase difference between the even and odd modes of light having the wavelength .lambda.B does not become .pi. when the light inputted through the port 75 is reflected by the diffraction grating 72 and returns to a terminal position 78 of the directional coupling section 71.
In the prior art, a distance Lb between the terminal position 78 of the directional coupling section 71 and the diffraction grating 72 is 2.8 mm, and the phase difference between the even and odd modes of light having the wavelength .lambda.B is 0.62.multidot..pi. when the light returns to the terminal position 78. In this case, 32% of the light is lost by reflection loss at the port 75.
As described above, the conventional optical circuit device using the directional coupling section 71 has satisfactory demultiplexing characteristics for demultiplexing of light having a specific wavelength, but has difficulty in multiplexing the wavelength again. This is because the diffraction grating is shifted from the center of the directional coupling section to obtain satisfactory demultiplexing characteristics.